|Hopefully you unplugged the power cord before changing the battery & grounded yourself to prevent a static discharge. If you zapped the board with static, you may have fried it.|
Anyhow, try this: with the power cord unplugged, move the Clear CMOS jumper from it's normal position to the 'clear' position....wait a few seconds, then move it back. Plug the power back in, then see if the system will boot.
As a side note, you need more RAM. 1GB isn't enough for Win7 to perform decently.